The present invention relates to steel tubes such as a sheath heater tube and black liquor heat recovery boiler tube, which are used under chloride-containing high temperature dry corrosion conditions.
That is, the present invention relates to a sheath heater steel tube exhibiting markedly improved resistance to dry corrosion at high temperatures. The present invention also relates to a black liquor heat recovery boiler tube for use in burning waste such as black liquor.
More particularly, the present invention relates to a tube which contacts a relatively concentrated chloride-containing substance or contaminants containing a relatively concentrated chloride under service conditions in a dry corrosion atmosphere.
Recently, in an increasing number of apartment complexes, the use of fuel gases is being restricted so as to avoid accidents due to gas leakage and reduce the possibility of fires at the time of earthquakes. Accordingly, electrical cooking appliances have been becoming increasingly popular.
Electrical cooking appliances such as cooking stoves and broilers for fish employ a sheath heater which generates heat at a maximum in the range of from 800.degree. to 900.degree. C. A sheath heater is a heater in which an electric heating element is embedded in an electrically insulating powder packed in a sheath, hereunder called a "sheath heater tube" or "sheath protector tube". For such a use, even the steel which resists oxidation under usual atmospheric conditions exhibits extremely poor resistance when it contacts soy sauce, mayonnaise, cooking salt or the like. This is because soy sauce usually contains 5% or more of NaCl. Thus, even if a protector tube for the sheath heater is made of a steel which exhibits generally good corrosion resistance, the sheath protector tube is easily attacked by dry chloride to cause the formation of pin holes, resulting in breakage of the heating elements.
Such high temperature corrosive conditions containing dry chlorides are found not only in electric cooking appliances, but also in incinerators for waste such as waste pulp liquor (black liquor), rubbish and the like.
Thus, a heat-exchanging boiler for use in burning waste pulp liquor cannot avoid contact with a 1% or more NaCl-containing atmosphere. A structural member of an incinerator, when a vinylchloride resin is burned, necessarily comes into contact with the HCl and Cl.sub.2 gases generated during combustion of the vinylchloride resin. Therefore, a means for achieving improved resistance to dry corrosion at high temperatures is urgently needed for these applications.
For a better understanding of the present invention, it is helpful herein to distinguish the atmosphere in which electric cooking appliances and incinerators mentioned above are used from that containing water, including high temperature or high pressure water. The former type is substantially free from liquid water, i.e. it is a dry corrosive environment, the corrosion mechanism of which is quite different from that of a so-called wet-corrosive environment. Namely, when a steel member is heated or is placed in a combustion gas in the presence of chlorides, oxidation as well as formation of sulfides occur, although the steel is totally free from stress-corrosion cracking or pitting which results in other severe problems in the presence of water.
When NaCl contacts a steel surface at a high temperature, the NaCl reacts with the Fe of the steel to form NaFeCl.sub.4 which is highly volatile and which accelerates dry corrosion. In addition, since free HCl and Cl.sub.2 form chlorides of Fe and Cr at a high temperature, corrosion is also accelerated. Furthermore, in an oxidizing atmosphere, the thus formed chloride then turns into an oxide, thus accelerating the dry corrosion through a corrosion cycle. Although under usual atmospheric conditions the once-formed Cr.sub.2 O.sub.3 layer exhibits protective duty and can resist oxidation, the presence of NaCl results in a porous oxide of (Fe, Cr).sub.2 O.sub.3 or (Fe, Cr).sub.3 O.sub.4, which is less resistant to oxidation.
Thus, means for achieving corrosion resistance under aqueous conditions cannot be directly applied to high temperature dry corrosion resistance in the presence of NaCl.
In the past it was reported that the addition of nickel is effective to improve corrosion resistance in a high temperature dry corrosive atmosphere containing chlorides. Therefore, as sheath heater tubes of electric cooking appliances, Incoloy 800 (Trademark for alloys of 21Cr-32Ni-Ti-Al-Bal. Fe), Incoloy 600 (Trademark for alloys of 15Cr-Bal. Ni), AISI 310S, 309S, and the like have been used. As boiler tubes for use in burning wastes, stainless steels such as AISI 321H or 304 have been used. In the form of bare tubes or coextruded tubes the steel mentioned above is used constituting an outer tube depending on the location in the boiler. In some cases, a cladding tube prepared by a metal spraying method is also used.
However, there is a decisive problem in these prior art materials that those containing a relatively large amount of nickel are very expensive, while those containing a small amount of nickel do not exhibit satisfactory properties. For example, if AISI 304 steel is used for boiler tubes for burning waste, it is required that the temperature of the boiler tube be restricted to lower than 500.degree. C. so as to lower the corrosion rate, which results in a decrease in thermal efficiency.
A metallic member used in these high temperature corrosive circumstances should exhibit not only improved resistance to corrosion in a high temperature dry corrosive atmosphere in the presence of chlorides, but also satisfactory high temperature strength, weldability, bending formability, and long-term stability of its chemical and physical properties. In view of these properties, the materials mentioned above have been selected for use in the past. However, the materials now available on the market are not satisfactory in respect to properties including resistance to corrosion under high temperature dry corrosive atmospheres.
In particular, a steel employed as a sheath heater tube must possess a uniform appearance and a high thermal radiation efficiency. Sometimes for the purposes of improving thermal radiation efficiency a black scale is formed on the surface by annealing. Therefore, the steel composition has to be so formulated that a satisfactory black scale can be easily formed during annealing.
Under these circumstances, a high temperature dry corrosion-resistant steel material which possesses all the above mentioned properties at satisfactory levels and is less expensive is highly desired.